The present invention relates to an image detection device for converting a light signal into an electrical signal and, more particularly, to an image detection device suitable for a medical X-ray diagnostic apparatus.
In recent years, an X-ray diagnostic apparatus using an image detection device including an a-Si TFT (amorphous silicon Thin Film Transistor) has been proposed in, e.g., U.S. Pat. No. 4,689,487. FIG. 13 is a block diagram showing the whole arrangement of this image detection device.
X-rays are emitted from an X-ray source 101 to be incident on an a-Si TFT image detection device 103 through an object 102 to be examined. The image detection device 103 generates and outputs an analog electrical signal corresponding to the quantity of X-rays having passed through it. The analog electrical signal is input to an A/D converter 109 in time-series and converted into a digital signal, and the digital signal is stored in an image memory 106. The image memory 106 stores image data of one or several images, and sequentially stores image data supplied to specific addresses on the basis of a control signal from a controller 105. The image data stored in the image memory 106 is extracted and processed by an arithmetic processor 110, and the result is re-stored in the image memory 106. The arithmetic result stored in the image memory 106 is converted into an analog signal by a D/A converter 107 and displayed as an X-ray image on an image monitor 108.
The a-Si TFT image detection device 103 has an arrangement as shown in FIG. 14. The TFT array is made up of a 2,000xc3x972,000 matrix of pixels (e1, 1) to (e2000, 2000). Individual pixels (ej, j) (j is an integer of 1 to 2,000) are parallel-connected at their two ends. One end of each pixel has a photoelectric conversion film 140 and pixel capacitor 142 to which a bias voltage from a power supply 148 is applied, and the other end has an a-Si TFT 144 having an input terminal connected to the other end of each of the photoelectric conversion film 140 and pixel capacitor 142, an output terminal connected to a signal line S1, and a gate connected to a scanning line G1.
When light is incident on the pixel, a current flows through the photoelectric conversion film 140 to accumulate charges in the capacitor 142. A scanning line driving circuit 152 drives the scanning lines G1 to turn on columns of the TFTs 144 whose gates are connected to themselves. Charges accumulated in the capacitor 142 having one end connected to the input terminal of each TFT 144 are transferred to an amplifier 154 via a signal line S1 connected to the output terminal of the TFT 144. The charge amount corresponds to a light quantity incident on the pixel, and the amplitude of an output signal from the amplifier 154 changes in accordance with the charge amount.
The output signal from the amplifier 154 is converted into a digital signal using an A/D converter (not shown) to display a digital image on a computer display. The pixel region shown in FIG. 14 has the same arrangement as a TFT liquid crystal display used in a compact information device such as a personal computer, and can be easily manufactured for a low-profile large display.
The arrangement shown in FIG. 14 has one TFT 144 per pixel. However, an actual device may have a plurality of TFTs 144 per pixel. For example, when charges accumulated in a given capacitor 142 are read out to a corresponding signal line S1 using a detective circuit having an arrangement as shown in FIG. 15, TFTs T1 and T2 are used. The TFT T1 is connected to the scanning line G1 at its gate and ON/OFF-controlled to output charges in the capacitor 142 to the signal line S1. The TFT T2 is connected between one end of the capacitor 142 and a ground terminal to operate as a protective diode.
Alternatively, a detective circuit shown in FIG. 16 is of an AMI (Amplified MOS Imager) type for converting charges into a voltage. The detective circuit includes TFTs T11 to T14, a constant current source made up of the TFTs T11 to T13 is connected between the capacitor 142 and the signal line S1, and the TFT T14 is connected as a reset transistor between one end of the capacitor 142 and a ground terminal. The gate of the TFT 14 receives a reset signal R1. The detective circuit shown in FIG. 15 directly reads out charges accumulated in the capacitor 142 to the signal line S1 via the TFT T1. To the contrary, the circuit shown in FIG. 16 converts charges in the capacitor 142 into a voltage to output the voltage.
High S/N ratios and wide dynamic ranges are required of X-ray image detection devices. For this reason, when a plurality of TFTs are formed in one pixel, their TFT characteristics must be made uniform. However, the TFT characteristics vary due to process variations. In particular, variations in OFF resistance and threshold voltage Vth degrade the image quality. Further, variations in OFF resistance increase the leakage current, resulting in large noise, a low S/N ratio, and a narrow dynamic range.
It is, therefore, an object of the present invention to provide an image detection device capable of realizing high image quality by suppressing the leakage current and increasing the S/N ratio.
An image detection device of the present invention comprises a signal line and scanning line which run perpendicularly to each other on a substrate, a pixel portion arranged at an intersection of the signal line and scanning line and including a photoelectric conversion film for converting incident light into a signal charge to accumulate the signal charge and a pixel electrode, a signal detective circuit including thin film transistors controlled in operation by the scanning line to read out a potential of the pixel electrode, and a scanning line driving circuit for driving the scanning line, wherein a thin film transistor having a source or drain connected to the pixel electrode out of the thin film transistors included in the signal detective circuit has an LDD structure on a high-potential side solely.
Even when pluralities of signal lines and scanning lines run on the substrate, pixel portions are arranged in a matrix at intersections of the signal lines and scanning lines, and the signal detective circuit includes thin film transistors controlled in operation by the respective scanning lines to read out potentials of corresponding pixel electrodes, the present invention can be applied. In this case, at least one thin film transistor having a source or drain connected to the pixel electrode out of the thin film transistors included in the signal detective circuit has an LDD structure on a high-potential side.
In the image detection device of the present invention, at least one transistor having a source or drain connected to the pixel electrode out of the thin film transistors included in the signal detective circuit has a multi-gate structure.
Alternatively, in the image detection device of the present invention, at least one transistor having a source or drain connected to the pixel electrode out of the thin film transistors included in the signal detective circuit has LDD structures on high- and low-potential sides in which an LDD length is larger on the high-potential side than the low-potential side.
When the signal detective circuit comprises a signal read transistor which has a drain or source connected to the pixel electrode, a source or drain connected to the signal line, and a gate connected to the scanning line, and is controlled in operation by the scanning line to output the potential of the pixel electrode to the signal line, and a protective transistor which has a drain or source and a gate connected to the pixel electrode, and a source or drain connected to a predetermined potential line, and connects the pixel electrode to the predetermined potential line when the potential of the pixel electrode reaches not less than a predetermined potential, the signal read transistor has an LDD structure on a high-potential side, and the protective transistor has an LDD structure on at least a high-potential side.
The signal read transistor and protective transistor can have a multi-gate structure.
Alternatively, the signal read transistor and protective transistor may have LDD structures on high- and low-potential sides in which the LDD-length is larger on the high-potential side than the low-potential side.
When the signal detective circuit comprises the signal read transistor, the protective transistor, and a threshold adjustment circuit including at least one transistor which receives a power supply voltage, has an input terminal connected to the pixel electrode and an output terminal connected to the gate of the protective transistor, and adjusts an operating threshold of the protective transistor in accordance with the potential of the pixel electrode, the signal read transistor and protective transistor have an LDD structure on a high-potential side.
The signal read transistor and protective transistor have a multi-gate structure, and the transistor included in the threshold adjustment circuit has a single-gate structure.
Alternatively, the signal read transistor and protective transistor have LDD structures on high- and low-potential sides in which the LDD length is larger on the high-potential side than the low-potential side.
When the signal detective circuit comprises the protective transistor, a reset transistor which has a drain or source connected to the pixel electrode, and a source or drain connected to a second predetermined potential terminal, and connects the pixel electrode to the second predetermined potential terminal when a gate receives a reset signal, and a voltage conversion circuit including at least one transistor which receives a power supply voltage, has an input terminal connected to the pixel electrode and an output terminal connected to the signal line, and generates a voltage signal corresponding to the potential of the pixel electrode to output the voltage signal to the signal line, the protective transistor and reset transistor have an LDD structure on a high-potential side.
The protective transistor and reset transistor have an LDD structure on a high-potential side.
The protective transistor and reset transistor have a multi-gate structure, and the transistor included in the voltage conversion circuit has a single-gate structure.
Alternatively, the protective transistor and reset transistor have LDD structures on high- and low-potential sides in which the LDD length is larger on the high-potential side than the low-potential side.
According to the image detection device, a TFT having an LDD structure on a high-potential side, a TFT having a multi-gate structure, or a TFT having LDD structures on high- and low-potential sides in which the LDD length is larger on the high-potential side than the low-potential side is employed for the signal read transistor having a source or drain connected to the pixel electrode. Thus, the image detection device can increase the OFF resistance to reduce the leakage current, and can prevent a decrease in S/N ratio owing to leakage of signal charges in the OFF state.
A TFT having an LDD structure on at least a high-potential side, a TFT having a multi-gate structure, or a TFT having LDD structures on high- and low-potential sides in which the LDD length is larger on the high-potential side than the low-potential side is employed for the protective transistor, thereby obtaining the same effects.
When a TFT having a single-gate structure is used for the transistor included in the threshold adjustment circuit or voltage conversion circuit, unlike the signal read transistor, protective transistor, or reset transistor, the drivability can be enhanced to increase the read sensitivity.